It was halfway through a conference presentation this week by Facebook that it dawned on me just how powerful the social network had become, just how big its own plans are and just how much it now has to lose.

Sure the rat-a-tat-tat artillery of stats sounded impressive: 400 plus million users worldwide, one third of the UK population active on Facebook each month with 50% of those returning each day, 100 million mobile users and, yes, 85 of the top 100 U.S. brands active on the network. But it wasn’t the numbers that made me sit up and pay attention, it was the confidence with which Facebook’s UK commercial director, Stephen Haines noted that major brands are starting to ditch their campaign microsites and make a home for ongoing customer engagement on Facebook because, to paraphrase, while microsites come and go Facebook was here to stay.

Now, the promise of longevity is a bold one coming from any Internet company. Facebook, of course, would seem to have plenty of reasons to be confident (its new moves to make the power of its social graph indispensable for the greater web merely underscoring its upbeat outlook) but social networking is a fickle business to say the least – just ask News Corporation and AOL, purchasers of the formerly high-flying MySpace and Bebo respectively.

In fact, the greater technology sector is rife with hubris. In 2000 AOL itself was primed to take over the online world after recording year-on-year membership growth just as fantastic as Facebook. Back in the late 1990s traditional media powerhouse Disney thought it had a smart portal play with Go.com until Yahoo showed it what a new media company was really about. And of course Yahoo itself was considered unassailable in search until Google and that algorithm took over.

I have no doubt that Facebook will doubt turn out to be more Google than Go.com but if recent history tells us anything it’s that there is no guaranteed long-term success in the Internet Economy (even Google knows this). This is especially true for Facebook, which prides itself in shaping business strategy based on the user habits of its members. For this reason alone major brands should think carefully before downing a jug of Facebook Kool Aid.

The current trend in social media business thinking is to invest more in the social places where people frequent (Facebook being a number one choice) rather than develop a brand specific social media platform or site that you then have to persuade the public to visit. When you see the interest Facebookers have in brand pages (Starbucks, for example, has seven million FB fans/likers) it’s not hard to see why major brand holders such as Unilever and Coca-Cola would prefer to interact with customers through Facebook and other social sites like YouTube rather than a campaign microsite with a limited shelf-life and even more limited consumer appeal.

This willingness to embrace change and release control suggests a fundamental shift in the way brands view social media. It tells us that what six months ago was seen as purely experimental has now become cemented as a fundamental part of customer engagement. Facebook, more than any social property, should pat itself on the back for making companies understand the power of customer clout. And yet, if it wants to be the go-to social destination for the next 10 years Facebook should also view the fickle power of customer choice (one it has harnessed so far) as a direct threat.

Facebook’s development announcements last week suggest the company understands that threat. By attempting to expand users’ Facebook experience outside of the network through its Social Plugins and Open Graph Protocol, the company is being blatant in its push to make a Facebook dominant part of everyone’s online life.

If it fails then you can be sure another new social network with news features will arise to woo the Facebook faithful. And when that happens the big brands will quickly flee to where their customers lead them. In the end, even Facebook friendship has its limits.